1. Field of the Invention
The invention relates generally to xenon short-arc ceramic lamps and specifically to such lamps which incorporate a spherical-elliptical reflector combination in a compound system to improve efficiency.
2. Description of the Prior Art
Short arc lamps provide intense point sources of light that allow light collection in reflectors for applications in medical endoscopes, instrumentation and video projection. Also, short arc lamps are used in industrial endoscopes, for example in the inspection of jet engine interiors. More recent applications have been in color television receiver projection systems.
A typical short arc lamp comprises an anode and a sharp-tipped cathode positioned along the longitudinal axis of a cylindrical, sealed concave chamber that contains xenon gas pressurized to several atmospheres. U.S. Pat. No. 5,721,465, issued Feb. 24, 1998, to Roy D. Roberts, describes such a typical short-arc lamp. A typical xenon arc lamp, such as the CERMAX marketed by ILC Technology (Sunnyvale, Calif.) has a three-legged strut system that holds the cathode electrode concentric to the lamp""s axis and in opposition to the anode.
U.S. Pat. No. 4,305,099, describes a light collection system for projectors, such as light valve projectors, which have a compound reflector associated with an arc lamp. The compound reflector includes an ellipsoidal reflector positioned to collect a portion of the light from the arc lamp and reflect a direct image of the light in a beam to an image forming plane of the projector and a spherical reflector positioned to collect another portion of the light from the arc lamp and reflect it back through the gap of the arc lamp to the ellipsoidal reflector to be reflected as a secondary image of the light from the lamp in the beam. The ellipsoidal and spherical reflectors are formed as full, uninterrupted surfaces of revolution. To provide uniform light distribution, the beam is directed through a pair of spaced lens plates, each having corresponding arrays, in rows and columns, of rectangular lenticules. The adjacent focus of the ellipsoidal reflector is centered in the arc, while the center of curvature of the spherical reflector, in order to avoid transmission loss through the arc, is displaced to a portion of the gap of the lamp which is relatively free of the arc. For maximum light efficiency, the direct image is focused just to one side, and the secondary image is focused just to the other side of the image forming plane. Such patents are all incorporated herein by reference.
Conventional lamps with parabolic collector/reflectors have the advantage of good collection and distribution efficiency when used in conjunction with a lens for focusing. However, such combinations can be too expensive for many applications. Conventional lamps with elliptical collector/reflectors have a different kind of problem. In order to collect a large polar angle of the lamp output, a wide spread of arc magnifications are automatically generated at the second focus. The rays with the smallest angles have the largest magnification. And the rays with the largest angles have the smallest magnification.
The collection efficiency of conventional elliptical collector/reflectors is good, but the distribution efficiency is often poor. In a compound reflector geometry that combines reflector types, the elliptical part is usually a rather shallow dish that provides a small spread of arc magnifications over a select spread of ray angles. But the polar angle collection of such a lamp""s output is rather poor from the sphere.
It is therefore an object of the present invention to provide a xenon ceramic lamp that is more efficient than conventional designs.
Briefly, an integrated compound reflector ceramic arc lamp embodiment of the present invention comprises three internal mirrors. Top and bottom concave mirrors encircle the inter-electrode gap. The third mirror is convex and is mounted coaxially on the stem of the cathode and faces a sapphire window. Its appearance is like that of a 360xc2x0 apron. Light rays emitted from the inter-electrode gap below a critical elevation angle will be reflected off the bottom concave mirror. Such rays bounce only once before exiting through the window to an external focus. Light rays emitted from the inter-electrode gap above the critical elevation angle, will be reflected off the top concave mirror. Such rays will bounce twice before exiting through the window to the focus. The rays that do reflect from the top concave mirror are directed to the convex cathode apron reflector, and from there will be reflected out the window to the focus.
An advantage of the present invention is that a ceramic lamp is provided in which the lamp collection angle is increased over the prior art.
Another advantage of the present invention is that a ceramic lamp is provided which is more efficient than the quartz lamps or other types of separate envelopes and compound reflectors.
A further advantage of the present invention is that a lamp is provided in which three mirror surfaces can be manipulated to control lumen delivery to an aperture, e.g., a light pipe for a projection television system.
A still further advantage of the present invention is that a lamp is provided in which reflected light is not depended upon to pass through the arc or near the arc, because such arc can actually block the passage.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment which is illustrated in the drawing figure.